1. Field of the Invention
This invention relates to a process for the preparation of an aqueous coating composition; an aqueous coating composition made by the process; a paint formulation; and an article coated with the coating composition or the paint formulation.
2. Description of the Related Art
The use of polymer coatings to protect surfaces, for example from moisture ingress, abrasion and erosion, and to enhance the aesthetic and functional properties of materials is known. These polymer coatings are typically applied to surfaces as liquid coating compositions using techniques such as rolling, brushing, sprinkling, casting and pneumatic or electrostatic spraying.
Often these coating compositions are adapted to exhibit rheological behaviour, that is they flow freely under high shear but are resistant to flowing when under low shear. In this manner the coating compositions have a consistency that is gel-like at rest, but fluid when agitated. This rheological behavior is referred to as thixotropy.
This thixotropic nature means that the coating compositions easily flow and level-out while under the forces associated with application to a surface, but resist the tendency to flow under lower shear forces such as gravity when applied to a vertical surface. This combines an easy applicability of the coating composition with a reduced tendency to sagging or dripping on vertical surfaces.
The use of rheology modification agents in conventional coating compositions to create a thixotropic profile and thus reduce sagging defects is widespread. Such rheology modification agents are commonly referred to as sag control agents.
The conventional thixotropic coating compositions have been predominantly based on solvent-borne polymer dispersions having high volatile organic compound (VOC) contents. For example, solvent-borne alkyds and oil-modified urethanes. A wide range of modifiers exist to impart the desired thixotropic behavior to such solvent-borne coating compositions, examples including modified inorganic clays and polyamide or polyurea modified alkyds.
Due to the need to reduce the emissions of VOCs into the earth's atmosphere, waterborne alternatives to the conventional solvent-borne alkyds and oil modified urethanes are needed. Waterborne thermoplastic vinyl polymer dispersions or thermoplastic waterborne polyurethane dispersions can be used in aqueous coating compositions but do not possess suitable performance properties such as chemical resistance, water resistance, blocking resistance and low temperature flexibility.
One particular group of water-borne polymer dispersions possessing a good combination of performance properties for use in aqueous coating compositions are those comprising carbonyl-hydrazide functional crosslinking systems. Examples of such compositions are described in patent applications U.S. Pat. No. 6,730,740 and EP1125949.
In U.S. Pat. No. 6,730,740, carbonyl-functional polymers are combined with polyfunctional hydrazide crosslinking agents in water-borne systems to form coating compositions having carbonyl-hydrazide crosslinking systems.
In such carbonyl-hydrazide crosslinking systems, crosslinking of the polymers takes place on drying through formation of a Schiff's base. Removal of water from the system, such as by evaporation during air drying, drives the reaction to the crosslinked product as illustrated in the below example scheme.

It is typical in aqueous coating compositions having carbonyl-hydrazide crosslinking systems that the carbonyl-functional groups are provided on the binder polymers, and the hydrazide-functional groups are provided on the crosslinkers. However, it is also possible that the carbonyl-functionality can be provided on the crosslinker and the hydrazide functionality can be provided on the binder polymer. Alternatively, both carbonyl and hydrazide functionality can be provided on the polymer binder allowing the molecules of the polymer binder to link directly to one another without the need for an additional crosslinker component.
Aqueous dispersions of coating compositions having carbonyl-hydrazide crosslinking systems provide useful aqueous coating compositions, and are desirable because of their low VOC requirements, and good performance properties.
However, obtaining the desired thixotropy in the aqueous coating compositions having carbonyl-hydrazide crosslinking systems is difficult using the commonly available sag control agents such as cellulosic ethers, alkali swellable acrylic polymers, hydrophobically modified alkali swellable thickeners or non-ionic associative thickeners. These thickeners provide pseudoplasticity rheology in a waterborne coating, in which viscosity decreases with increasing rate of shear (also termed shear thinning). Introducing pseudoplasticity, a decrease in viscosity over time at a constant shear rate is much more complex. Examples of thixotropic agents that can be used in waterborne paints to obtain such pseudoplasticity include fumed silicas and modified clays. However, the addition of these substances often has an adverse effect on coating properties such as gloss or film-clarity.
There is thus a need for sag control agents that show good results for aqueous coating compositions having carbonyl-hydrazide crosslinking systems.
WO 2008/030445 and WO 2008/030437 describe aqueous compositions of polyurea sag control agents obtained by reacting hydrophilic polyisocyanates with a primary amino-functional compound having at least one hydroxyl functional or ether group. The polyurea compounds are prepared directly in water, optionally containing a water-miscible organic solvent, and are then added to a coating composition. Alternatively they can be prepared “in-situ”, in the presence of the binder and crosslinking system. The sag control agents of these patent applications are not disclosed for use with coating compositions having carbonyl-hydrazide crosslinking systems.
It is an object of the present invention to provide a sag control agent for aqueous coating formulations having carbonyl-hydrazide crosslinking systems, in particular the sag control agent being formed in situ with the carbonyl-hydrazide crosslinking system.